[Beav]

As a 'rule of thumb' p/s fluid can replace ATF in power steering but ATF cannot be used in place of p/s fluid. ATF has a great deal of friction modifiers, etc. whereas p/s fluid is a basic hydraulic oil. Some systems contain somewhat softer aluminum parts and the friction modifiers can/will cause premature wear. Also the seals may take a toll as well.

Probably the reason for the increased assist was that your old fluid was burnt/dirty (read: loss of viscosity due to thermal break-down.) I routinely do a handful of p/s flushes each week and it is easy to notice the decrease in effort as the fluid is replaced. The system we use sucks out the old fluid and inserts a flush solution. This process is repeated several times with a cycling of the steering wheel lock-to-lock between each drain and refill. This ensures total removal and cleaning of the components. After the flush solution is finished the process is repeated, this time replacing the flush with fresh fluid, again several times with cycling lock-to-lock. A simple drain and refill is better than nothing but nowhere as good or complete. You should see the nasty fluid I remove from older systems.